1. Field of the Invention
The present invention generally relates to refractory castables, and in particular to refractory castables fabricated using aggregate coated with a hydrophobic component.
2. Description of the Background
Refractory castables (a.k.a. heat-resistant concretes) are composed of a heat-resistant aggregate and a heat-resistant hydraulic cement. Generally, refractory castables are formed from ground refractory materials containing a suitable percentage of added bonding agents. Refractory castables range from temperature-graded aggregates and hydraulic cements in proportions formulated to achieve desired properties for the particular end use. In practice, refractory castable mixes require only the addition of clean water during their application, for example, during pneumatic gunning, to form a heat resistant concrete piece or structure. Such refractory castables are utilized in a variety of industries, including metal foundries (e.g., iron, steel, and alumina), hydrocarbon processing, power generation, and mineral processing. Refractory castables are generally engineered to possesses specific properties (e.g., heat resistance, shrinkage, insulating capability, density) depending upon the particular application.
Refractory castable components, including aggregates, are generally shipped in dry form, blended with water, and then poured, cast, tamped, or applied in place by various means. For example, one means of applying refractory castables in place involves spraying the material (via either a dry or wet mix process) through the use of an air gun. The dry mix method involves placing the dry ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle, then adding water at the nozzle as the material is impelled to the receiving surface. Mixing of the dry components with water is completed as the mixture hits the receiving surface. Conversely, the wet mix process involves pumping a previously prepared concrete (i.e., dry components already mixed with water) to the nozzle and introducing compressed air at the nozzle to impel the mixture onto the receiving surface. This wet mix application process is also referred to as “gunning” and castables applied in this manner are often referred to as “gunned” as opposed to cast.
Refractory castables are especially suited for furnace linings of irregular dimensions, for patching brickwork, and for applications that possess special shapes. Refractory castables are commonly used to line a heated chamber to provide heat insulation, thereby reducing heat loss from the chamber and increasing the efficiency of the overall process.
A considerable amount of work has been done over the years to improve the properties of refractory castables. For example, much work has been done to lower the water content required for casting refractory castables. For prior art systems, reduction in water used in casting leads to higher density castables having lower porosity. While these attributes are beneficial for thermal insulation properties, they also represent an engineering challenge as denser castables place a heavier load on the structures in which they are placed or applied.
One strategy for reducing the water content required for casting refractory castables is the use of dispersants and ultrafine particles in combination with particle packing principles. The dispersants minimize formation of flocs that raise water requirements. More optimum particle packing has been achieved by using progressively finer particles to fill in the voids between the coarser aggregates. Without these ultrafine particles, the voids between the coarser aggregates would fill with water during casting, thus, increasing the amount of water needed in the refractory castable. It will be appreciated by those skilled in the art that a substantial reduction in the amount of water required for casting refractory castables was realized with the discovery that ultrafine particles (i.e. particles having a diameter of less than about three microns) could be included in the products when used with appropriate dispersants. The ultrafine particles used for this purpose have been ultrafine refractory materials, namely, oxides, such as for example, microsilica and finely ground alumina being the most common choices. European Patent no. 0742416 discloses a spray operation method for monolithic refractories wherein a fine powder of alumina or fumed silica having a particle size of at most three microns imparts good flowability to the mixed batch of monolithic refractories. U.S. Pat. No. 5,549,745 and U.S. Pat. No. 5,512,325 disclose a non-slumping, high density, low moisture, low cement sprayable refractory castable composition which can be applied without forms containing a solid flow aid that is microsilica, 1 to 3 micron alumina, or mixtures thereof.
While these approaches achieve lower water requirements for casting, they also introduce drawbacks, particularly as relating to the use of microsilica and ultrafine alumina. For example, microsilica typically reduces high temperature refractoriness of refractory castables. In high alumina and fireclay castables bonded with calcium aluminate cement, for example, the microsilica combines with lime from the calcium aluminate cement when the castable is heated, forming low melting point glasses. These glasses may introduce problems such as making the castable more prone to creep at high temperatures, decreasing the hot strength, and increasing the susceptibility of the castable to chemical corrosion. In an attempt to deal with the problem of glass formation, refractory compositions with either no calcium aluminate cement, or reduced levels of calcium aluminate cement have been developed. The resulting lower lime contents reduce the amount of low melting glass that forms, but refractoriness is still not optimized because the microsilica in the matrix of the castable remain susceptible to chemical alteration and fluxing by constituents commonly found in the environments in which the castables were used.
Further, ultrafine alumina, when used to reduce the water required for casting refractory castables, is a substantial commercial impediment because of its high cost. In addition, ultrafine alumina can have a detrimental effect on the rheology of refractory castables, in particular those that are bonded with calcium aluminate cement. Refractory castables containing calcium aluminate cement and ultrafine alumina can exhibit short working times and poor casting characteristics. While not fully understood, it is believed that the ultrafine alumina provides nucleation sites for precipitation of hydrate phases from solution during mixing and placement of the castables. It is known by those skilled in the art that alumina-lime hydrate phases form on the edges of ultrafine alumina particles in suspensions of ultrafine alumina and calcium aluminate cement in water. It is believed that these hydrate phases affect the morphology of the finest constituents in the refractory castables, thereby adversely affecting rheology and casting characteristics.
Other ultrafine refractory oxides for reducing the amount of water required for casting refractory castables have similar drawbacks. For example, ultrafine chromic oxide is expensive and is undesirable from an environmental standpoint. Ultrafine titanium is also expensive and is generally regarded as a flux in refractory systems. Thus, despite the ability of various ultrafine refractory oxides to reduce the water required for casting refractory castables, no ultrafine refractory material heretofore has been found that is wholly satisfactory from either a technical, environmental, or economic standpoint. Further, it will be appreciated by those skilled in the art, that no ultrafine refractory material heretofore has been found that provides for reducing the amount of conventional ultrafine refractory oxides employed or for eliminating the use of conventional ultrafine refractory oxides to fill in the voids between the coarser refractory aggregates while at the same time maintaining particle packing principles for reducing the water content required for casting.
An additional challenge facing the refractory castable industry is the widespread problem of alkaline hydrolysis. In the presence of water, the calcium aluminate hydrate commonly found in refractory castables may react with alkali components of the castable. Through hydrolysis, the calcium aluminate hydrate breaks down into calcium carbonate and aluminum trihydrate. Depending on the atmospheric humidity and other potential sources of water, alkaline hydrolysis progresses slowly resulting initially in a thin soft surface on the castable. As time goes on, alkaline hydrolysis occurs at greater depths into the surface resulting in significant spalling of the castable and a concomitant degradation in the thermal insulation properties of the castable. When that physical and thermodynamic degradation progresses significantly, the operational life span of the castable is diminished and castable must be replaced. This, of course, requires shutdown of the industrial equipment to allow replacement of the damaged castables or, in some circumstances, replacement of the entire castable ensemble.
To slow down the alkaline hydrolysis reaction, numerous strategies are employed. For example, castable component may be dried extensively prior to shipping to the final plant destinations. The dryness of the castable must be maintained; if moisture is introduced to the dried castable, water penetrates the dry material quickly and alkaline hydrolysis will occur, leading to degradation of large portions of the castable. Another strategy is to apply organic coatings to the surface of the castable. This practice carries additional labor and material costs, and requires careful consideration of the potential of reaction of the organic coating with the interior environment of the industrial equipment that the castables line or coat. Further, the organic coating loses its effectiveness if physically disrupted.
Accordingly, there remains a very real and substantial need for a refractory composition, castable, and spray mix capable of lowering the amount of water required for casting or spraying the refractory castable composition while at the same time reducing or eliminating the undesirable characteristics known to exist in the prior art. These castables should possess exemplary physical and thermodynamic properties and would preferably confront many of the known challenges to castable industry. The present invention addresses this need.